1. Field of the Invention
The present invention relates to the field of antennas. Specifically, it relates to the control (including beam and null steering and tuning) of phased arrays and subarrays by using parasitic control elements in the aperture of each individual antenna element in the array.
2. Related Art
Array antennas refer to the class of antennas which are formed by phase-coherent combining of the outputs from multiple stationary antenna elements. The array antenna's spatial beam pointing characteristics are determined by the positions of the individual radiators (elements) and the amplitudes, phases, and time-delays of their radiation. The amplitudes, phases, and time-delays are, in general, controlled by the excitation of the individual antenna elements within the array. The antenna array characteristics are also controlled, and usually limited, by the properties of the individual elements in the antenna array and the way in which they interact with each other. These properties include the frequency properties of the individual elements as well as the element gain patterns.
In many cases, antenna arrays are subdivided into smaller arrays known as subarrays. Subarrays, when used, are constructed for ease of mechanical construction as well as for providing a way to minimize the amount of feed and phase control or time-delay structure needed to control the radiation of the individual elements.
The versatility of phased array antennas is convincingly evidenced by their broad range of military and commercial applications including communications, radar, and electronic countermeasures. For all of their advantages, however, performance of phased array antennas is limited. Phased arrays are usually limited in the range of angles over which they can effectively steer a beam without significant losses in overall system gain.
There are two primary reasons for this. These can be seen by first considering an array made up of N individual antenna elements. The overall array gain. G(2), for this N-element system in the direction θ when each element is uniformly illuminated, can be written (in dBi) as,G(θ)=10 log(N)+g(θ)+10 log(1−|Γ(θ)|2)−α  (1)
where g(θ) is the embedded gain pattern for an individual antenna element in the direction θ, Γ(θ) is the active reflection coefficient, and α represents losses in the beam forming network that are independent of Γ(θ). Equation (1) presumes the array is being steered in the θ direction. Thus, the expression does not represent the array pattern but, rather, it represents the gain in the scan direction. In particular Γ(θ) is the effective reflection coefficient when the individual array elements are phased so as to produce a beam in the θ direction. Also g(θ) and Γ(θ) are assumed to be the same for all of the N elements. This latter assumption is an approximation. In practice there will be element-to-element differences. Indeed, for applications involving moderate sized arrays the element-to-element variations in g(θ) may have significant impacts on the array performance.
However, for this example we consider (1) to be a reasonably accurate summary of the array characteristics. Suppose Ĝ is the desired system gain given by a requirement or specification. This specification can be met at angles for which,g(θ)+10 log(1−|Γ(θ)|2)≧Ĝ−10 log(N)+α  (2)If g(θ) gets too small and/or |Γ(θ)| gets too close to 1, then condition (2) cannot be satisfied. Thus, presuming that the losses α are acceptable, drop off in g(θ) and increase in |Γ(θ)| are the two basic factors that limit the coverage of the array. For certain steer angles the mutual coupling among the elements can become substantial. This leads to an increase in |Γ(θ)| and, consequently, the system may not be able to meet specifications for that range of angles.
In other words, the embedded element gain characteristics as a function of angle and frequency are fundamental limits in the range of angles the array can be scanned to and the frequencies at which it will operate.
Introducing reconfigurability at the individual antenna element level leads to the possibility of much higher performance in gain, pattern shaping, and frequency agility of the individual array elements which then leads to enhanced overall array performance. The purpose of reconfiguration to the array is to adapt the gain characteristics of the individual antenna elements so as to get the maximum possible performance from the antenna system.
Reconfiguration can be used to expand the range of operational functions in a number of ways. First, the embedded array elements can be frequency tuned, and bandwidth can be improved by using reconfiguration to broaden the bandwidth of the embedded elements. In addition, for high gain arrays, beam squint is usually the limiting factor on instantaneous bandwidth. Reconfiguration can alleviate this problem by providing control of the element phase centers. Scan coverage can be improved and scan blindness alleviated by controlling the embedded antenna gain patterns of the elements as well as by providing control of the active impedance as the beam is scanned. Applying limited phase control to the elements themselves can alleviate some of the complexity of the feed manifold.